Figure 9.13: Comparison of the distribution of acid rain in the United States from 2001 to 2006. Measurements are taken from the annual sulfate deposition, derived from airborne sulfur oxides (SO2). Deposition levels of 18 pounds per acre (20 kilograms per hectare) are generally regarded as threatening to some aquatic and terrestrial ecosystems. Notice that the intensity of acid rain has decreased in some places and increased in others.
(Source: National Atmospheric Deposition Program web site, http://nadp.sws.uiuc.edu/.)